High-cr ferritic/martensitic steel having improved creep resistance and preparation method thereof

ABSTRACT

High-Cr ferritic/martensitic steels having an improved tensile strength and creep resistance are provided, which includes 0.04˜0.13 weight % of carbon, 0.03˜0.07 weight % of silicon, 0.40˜0.50 weight % of manganese, 0.40˜0.50 weight % of nickel, 8.5˜9.5 weight % of chromium, 0.45˜0.55 weight % of molybdenum, 0.10˜0.25 weight % of vanadium, 0.02˜0.10 weight % of tantalum, 0.15˜0.25 weight % of niobium, 1.5˜3.0 weight % of tungsten, 0.05˜0.12 weight % of nitrogen, 0.004˜0.008 weight % of boron, and optionally, 0.002˜0.010 weight % of phosphorus or 0.01˜0.08 weight % of zirconium, and iron balance. By regulating the contents of alloying elements such as niobium, tantalum, tungsten, nitrogen, boron, zirconium, carbon, the high-Cr ferritic/martensitic steels with superior tensile strength and creep resistance are provided, and can be effectively used as an in-core structural material for Generation IV sodium-cooled fast reactor (SFR) which is used under high temperature and high irradiation conditions.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from Korean Patent Application No. 10-2010-0001425, filed on Jan. 7, 2010, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to high-Cr ferritic/martensitic steel having improved creep resistance and a preparation method thereof.

2. Description of the Related Art

The Sodium-Cooled Fast Reactor (SFR) uses a fast neutron, and has nuclear fuel breeding characteristic. Accordingly, since the early stage of nuclear power industry, SFR has been continuously developed mainly for efficient use of uranium resources. As reflected in the Generation IV reactor (Gen IV) development program, the sodium-cooled fast reactor has regained the spotlight for recycling of used nuclear fuels and transmutation of long-lived radionuclide wastes.

Nuclear fuel is an essential element of sodium-cooled fast reactor in which processing such as nuclear fission for energy generation, energy breeding from nuclear material or transmutation of nuclear waste is performed. Therefore, the stability of nuclear fuel in which radioactive nuclear fission products are concentrated is directly related to the stability of nuclear reactor.

Since a nuclear fuel cladding tube seals core and prevents radioactive materials from leaking, the nuclear fuel cladding tube is the most important nuclear fuel component which is directly related to the stability of nuclear fuel and a nuclear reactor. The nuclear fuel cladding tube of SFR is designed to use in severe conditions of high temperature and high irradiance. Therefore, a cladding tube having superior mechanical properties and excellent irradiation resistance at these severe environments should be developed for a safe operation for prolonged periods.

Thus, Ferrite/Martensitic Steel (FMS) which has superior mechanical/irradiation properties at a high temperature has drawn wide attention as a candidate material for major core components in Generation IV reactor and nuclear fusion reactor.

The FM steel which includes 8˜12 weight % of chromium has been used as a material for the in-core components of the fast breeder reactor including a nuclear fuel cladding tube, a wrapper which wraps the nuclear fuel cladding tube, or a duct, because FMS has the superior thermal properties and irradiant swelling resistance compared to austenitic stainless steels (e.g., SS316, SS304). Alloy HT9, which was originally developed as a heatproof material for thermal power station (main elements: 12% Cr—1% Mo—0.5% W—0.3% V), was selected as a material of nuclear fuel cladding tube and duct for Experimental Breeder Reactor-II (EBR-II). Also, in Europe and Japan, ferrite/martensitic steels were primarily selected as cladding tube materials of fast reactor. In recent, 9 weight % Cr FM steel has been considered as a core structural material for a highly efficient generation IV reactor under the high temperature and radioactive environments exceeding 600° C. and 200 dpa, respectively.

In the mid 1980s, material development program of nuclear fusion reactor has begun to develop in earnest, and the concept of reduced-activation steel was introduced. In such a circumstance, the study of low radioactive FM steel (RAFMS) was actively conducted, starting with the material such as FM steel of ASTM GR.91 alloy (main elements: 9% Cr—1% Mo—0.20% V—0.08% Nb), which is well known as modified 9Cr-1 Mo steel. The low radioactive FM steel has the limited options in terms of the alloying elements added to reduce long-lived high level radioactive material generated by fast neutron irradiation. That is, addition of molybdenum, niobium, nickel, copper, nitrogen to low radioactive FM steel was strictly limited. Instead, adding tungsten and tantalum to low radioactive FM steel was suggested. Also, alloy with 7˜9% reduced chromium is preferred as a way of inhibiting the generation of δ-ferrite steel which is bad influence on impact properties without increasing addition of carbon or manganese which is α-phase stabilizing element. With these series of researches, F82H alloy (main elements: 8% Cr—2.0% W—0.25% V—0.04% Ta) and JLF-1 alloy (main elements: 9% Cr—2.0% W—0.25% V—0.05% Ta—0.02% Ti) from Japan, EUROFER-97 alloy (main elements: 9% Cr—1.1% W—0.20% V—0.12% Ta—0.01% Ti) from Europe, ORNL 9Cr-2WVTa (main elements: 9% Cr—2.0% W—0.25% V—0.07% Ta) from America have been newly developed.

Yet, since a SFR nuclear cladding tube is used under the severe condition such as high temperature and irradiation of fast neutron, it is still necessary to develop a high-Cr ferritic/martensitic steel having an improved creep resistance.

To develop a high-Cr ferritic/martensitic steel having improved tensile strength and creep resistance at high temperatures, the contents of alloy elements of niobium, tantalum, tungsten, nitrogen, boron, zirconium, carbon or the like have been adjusted and thus achieved in the present invention.

SUMMARY OF THE INVENTION

Exemplary embodiments overcome the above disadvantages and other disadvantages not described above. Also, the embodiments are not required to overcome the disadvantages described above, and an exemplary embodiment of the present invention may not overcome any of the problems described above.

In one embodiment, a high-Cr ferritic/martensitic steel having an improved creep resistance as a core material for a sodium-cooled fast reactor (SFR) is provided.

To achieve the above-mentioned objects of the present invention, in one embodiment, a high-Cr ferritic/martensitic steel having improved tensile strength and creep resistance as a material for a sodium-cooled fast reactor (SFR) may include 0.04˜0.13 weight % of carbon, 0.03˜0.07 weight % of silicon, 0.40˜0.50 weight % of manganese, 0.40˜0.50 weight % of nickel, 8.5˜9.5 weight % of chromium, 0.45˜0.55 weight % of molybdenum, 0.10˜0.25 weight % of vanadium, 0.02˜0.10 weight % of tantalum, 0.15˜0.25 weight % of niobium, 1.5˜3.0 weight % of tungsten, 0.05˜0.12 weight % of nitrogen, 0.004˜0.008 weight % of boron and iron balance.

In another embodiment, a high-Cr ferritic/martensitic steel having improved tensile strength and creep resistance as a material for a sodium-cooled fast reactor (SFR) may include 0.04˜0.13 weight % of carbon, 0.03˜0.07 weight % of silicon, 0.40˜0.50 weight % of manganese, 0.40˜0.50 weight % of nickel, 8.5˜9.5 weight % of chromium, 0.45˜0.55 weight % of molybdenum, 0.10˜0.25 weight % of vanadium, 0.02˜0.10 weight % of tantalum, 0.15˜0.25 weight % of niobium, 1.5˜3.0 weight % of tungsten, 0.05˜0.12 weight % of nitrogen, 0.004˜0.008 weight % of boron, and iron balance, and further include 0.002˜0.010 weight % of phosphorus, or 0.01˜0.08 weight % of zirconium.

Since high-Cr ferritic/martensitic steel shows improved tensile strength and creep resistance due to adjusted contents of niobium, tantalum, tungsten, nitrogen, boron, zirconium, or carbon, the high-Cr ferritic/martensitic steel can be effectively used as a nuclear material for Generation IV SFR nuclear fuel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation of yield strength and tensile strength of high-Cr ferritic/martensitic steels at 650° C. according to an embodiment; and

FIG. 2 is a graphical representation of creep resistance of high-Cr ferritic/martensitic steels at 650° C. according to an embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The present invention will be explained in greater detail below.

A high-Cr ferritic/martensitic steels according to an embodiment may include 0.04˜0.13 weight % of carbon, 0.03˜0.07 weight % of silicon, 0.40˜0.50 weight % of manganese, 0.40˜0.50 weight % of nickel, 8.5˜9.5 weight % of chromium, 0.45˜0.55 weight % of molybdenum, 0.10˜0.25 weight % of vanadium, 0.02˜0.10 weight % of tantalum, 0.15˜0.25 weight % of niobium, 1.5˜3.0 weight % of tungsten, 0.05˜0.12 weight % of nitrogen, 0.004˜0.008 weight % of boron and iron balance.

Also, the high-Cr ferritic/martensitic steel may further include phosphorus or zirconium. To be specific, 0.002˜0.010 weight % of phosphorus or 0.01˜0.08 weight % of zirconium may be included.

The followings are the functions and effects of respective elements added according to an embodiment.

(1) Carbon (C)

In a high-Cr ferritic/martensitic steel according to an embodiment, carbon forms carbide to provide precipitation hardening effect. Preferably, the carbon is contained in amount of 0.04˜0.13 weight %. If the amount of the carbon is less than 0.04 weight %, the mechanical strength deteriorates at a room temperature and toughness also deteriorates. To be specific, delta ferrite is generated due to increasing Cr equivalent. If the amount of carbon exceeds 0.13 weight %, many carbides are generated, and strengthening effect of precipitates degrades since such carbides are easily coarsened during use.

(2) Silicon (Si)

In a high-Cr ferritic/martensitic steel according to an embodiment, silicon improves oxidation resistance, and is used as a deoxidant in steel manufacturing. Preferable content of silicon is 0.03˜0.07 weight %. If the amount of silicon is less than 0.03 weight %, corrosion resistance deteriorates, and if the amount of silicon exceeds 0.07 weight %, generation of laves phase is promoted, thereby degrading toughness.

(3) Manganese (Mn)

In a high-Cr ferritic/martensitic steel according to an embodiment, manganese promotes hardenability. Preferable content of manganese is 0.40˜0.50 weight %. If the amount of manganese is less than 0.40 weight %, there is a problem associated with hardenability, and if the amount of manganese exceeds 0.50 weight %, creep resistance deteriorates.

(4) Nickel (Ni)

In a high-Cr ferritic/martensitic steel according to an embodiment, nickel suppresses generation of delta ferrite by increasing the chromium (Cr) equivalent. Preferable content of nickel is 0.40˜0.50 weight %. If the amount of nickel is less than 0.40 weight %, delta ferrite which is weak in toughness is generated, and if the amount of nickel exceeds 0.50 weight %, as in the case of manganese, creep resistance degrades.

(5) Chromium (Cr)

In a high-Cr ferritic/martensitic steel according to an embodiment, chromium enhances corrosion resistance and high-temperature strength. Preferable content of chromium is 8.5˜9.5 weight %. If the amount of chromium is less than 8.5 weight %, resistance against high temperature oxidation and corrosion degrades, and if the amount of chromium exceeds 9.5 weight %, creep resistance degrades.

(6) Molybdenum (Mo)

In a high-Cr ferritic/martensitic steel according to an embodiment, molybdenum has solid-solution hardening effect. Preferable content of molybdenum is 0.45˜0.55 weight %. Since the molybdenum content is co-related with the tungsten content. That is, if the amount of molybdenum in a steel containing tungsten is less than 0.45 weight %, the amount of chromium decreases and delta ferrite is generated, and if the amount of molybdenum exceeds 0.55 weight %, laves phase which has brittleness are generated massively.

(7) Vanadium (V)

In a high-Cr ferritic/martensitic steel according to an embodiment, vanadium is an alloying element exhibiting precipitate hardening. Preferable content of vanadium is 0.1˜0.25 weight %. If the amount of the vanadium is less than 0.1 weight %, creep resistance deteriorates since the sites where precipitates are generated decrease, which is causing irregular distribution of carbides, and form coarse carbides. If the amount of vanadium exceeds 0.25 weight %, all the solid carbon and nitrogen on a matrix are consumed, and other forms of carbides are hardly generated during use.

(8) Niobium (Nb)

In a high-Cr ferritic/martensitic steel according to an embodiment, niobium is an alloying element exhibiting precipitate hardening. Preferable content of niobium is 0.15˜0.25 weight %. If the amount of niobium is less than 0.15 weight %, niobium precipitates are not sufficiently generated, causing austenitic grain growth during normalizing treatment, thereby deteriorating the mechanical performance. If the amount of niobium exceeds 0.25 weight %, the non-solid niobium content increases, decreasing the vanadium precipitates which are effective for creep resistance, and consuming solid carbons in a matrix, thereby reducing the carbide precipitates such as M23C6 and eventually decreasing the long-term creep resistance.

(9) Tantalum (Ta)

In a high-Cr ferritic/martensitic steel according to an embodiment, tantalum is a low radioactive element and has precipitation hardening effect when contained in niobium precipitates. To achieve the superior mechanical properties according to an embodiment, the tantalum is contained preferably in amount of 0.02˜0.10 weight %. If the amount of tantalum exceeds 0.10 weight %, the same problem is experienced as in the case of adding an excessive amount of niobium.

(10) Tungsten (W)

In a high-Cr ferritic/martensitic steel according to an embodiment, tungsten is representative solid-solution hardening alloying element. Preferable content of tungsten is 1.5˜3.0 weight %. If the amount of tungsten is less than 1.5 weight %, effective solid-solution hardening can not be obtained, and if the amount of tungsten exceeds 3.0 weight %, laves phase, which is known to degenerate long-term creep resistance and toughness, is generated.

(11) Nitrogen (N)

In a high-Cr ferritic/martensitic steel according to an embodiment, nitrogen forms nitride or solidifies interstitial form to increase the strength. Preferable content of nitrogen is 0.05˜0.12 weight %. If the amount of the nitrogen is less than 0.05 weight %, corrosion resistance degrades, and if the amount of nitrogen exceeds 0.12 weight %, creep resistance degrades rapidly.

(12) Boron (B)

In a high-Cr ferritic/martensitic steel according to an embodiment, boron segregates boundaries and reinforces boundaries to enhance creep resistance at a high temperature. Preferable content of boron is 0.004˜0.008 weight %. If the amount of boron is less than 0.004 weight %, effective boundary enforcement cannot be achieved, and if the amount of boron exceeds 0.008 weight %, boron precipitates are generated which rather degrades the creep resistance and causes problem in production.

(13) Phosphorus (P) or Zirconium (Zr)

In a high-Cr ferritic/martensitic steel according to an embodiment, a small amount of phosphorus or zirconium can be additionally included to improve the creep resistance.

Preferably, the amount of added phosphorus is 0.002˜0.010 weight %, and the amount of zirconium is 0.01˜0.08 weight %. If the content exceeds the above-mentioned range, the mechanical properties rather deteriorate.

A high-Cr ferritic/martensitic steel according to an embodiment can be achieved by the conventionally known method which may preferably include:

mixing and dissolving alloying elements to prepare an ingot (step 1):

Hot working such as hot forging, hot extrusion and hot rolling the prepared ingot of step 1 (step 2):

normalizing and air cooling the hot worked ingot of step 2 (step 3): and

tempering and then air cooling the normalized alloy of step 3 to prepare a high-Cr ferrite/martensitic steel (step 4).

To produce a high-Cr ferritic/martensitic steel according to an embodiment into required forms for nuclear fuel devices (such as a nuclear fuel cladding tube or duct of a sodium-cooled fast reactor), after the tempering of step 3 above, steps of heat treating and cold working may additionally be performed several times and then final heat treatment step may be performed.

Respective steps of the preparation method according to an embodiment will be described below.

At step 1, an ingot is prepared by mixing and dissolving alloying elements.

The alloying elements may use carbon, silicon, manganese, nickel, chromium, vanadium, tantalum, niobium, tungsten, nitrogen, boron, and iron balance, and to be specific, may include 0.04˜0.13 weight % of carbon, 0.03˜0.07 weight % of silicon, 0.40˜0.50 weight % of manganese, 0.40˜0.50 weight % of nickel, 8.5˜9.5 weight % of chromium, 0.45˜0.55 weight % of molybdenum, 0.10˜0.25 weight % of vanadium, 0.02˜0.10 weight % of tantalum, 0.15˜0.25 weight % of niobium, 1.5˜3.0 weight % of tungsten, 0.05˜0.12 weight % of nitrogen, 0.004˜0.008 weight % of boron and iron balance.

Additionally, phosphorus or zirconium may be included. To be specific, 0.002˜0.010 weight % of phosphorus, and 0.01˜0.08 weight % of zirconiummay be additionally included.

The ingot may be prepared by vacuum inducing melting (VIM) method.

To be specific, in a melting chamber, alloy elements may be dissolved under the atmosphere of high vacuum (1×10⁻⁵ to 0.5 torr) with induced currents applied, and deoxidant such as aluminum or silicon is introduced. At a point when melting almost finishes, micro-elements, especially nitrogen may be charged into the melting chamber and a sample for componential analysis is collected. After the melting is completed, outflow is carried out in which the molten metal is poured into a rectangular mold, and an oxidized layer of the surface was mechanically processed to prepare the ingot.

At step 2, the ingot prepared at step 1 is hot worked.

Through the hot working such as hot forging, hot extrusion and hot rolling, hot working product which is suitable for hot working is prepared. The hot rolling is desirably performed at 1100˜1200° C. for 0.5˜2 hours. In case the above-mentioned conditions are not satisfied, for example, if the temperature is less than 1100° C., the purpose of solution annealing is not satisfactorily achieved, and if the temperature exceeds 1200° C., the grain size of prior-γ phase grows too excessively to degrade the mechanical properties of the final product.

At step 3, hot worked product at step 2 is normalized and air-cooled.

The normalizing is desirably performed at the γ-phase temperature of 1000˜1100° C. for 0.5˜2 hours to re-dissolve the precipitate phase which is unnecessarily produced on the hot worked product, and to regulate the cooling temperature to thus control the size and amount of the precipitates.

At step 4, the normalized alloy at step 3 is tempered and air-cooled to form a high-Cr ferritic/martensitic steel.

The tempering is desirably performed at 600˜800° C. for 1˜3 hours to produce stable, minute and uniform precipitates

With the preparation method explained above, a high-Cr ferritic/martensitic steel is produced.

Furthermore, to prepare a high-Cr ferritic/martensitic steel according to an embodiment as a device for SFR nuclear fuel, after the tempering of step 3 above, steps of heat treating and cold working may be additionally performed several times and then step of final heat treatment may be performed.

To be specific, the additional heat treating may be performed at 600˜800° C. for 1˜3 hours, cold working may be performed for 2˜4 times, and final heat treating may be performed at 600˜800° C. for 1˜3 hours to produce a high-Cr ferritic/martensitic steel.

A high-Cr ferritic/martensitic steel prepared according to the preparation method explained above have superior tensile strength of 330˜372 MPa at a high temperature of 650° C., and also superior creep resistance at 140 MPa stress intensity for 1833˜3233 hours of rupture time. Since the high-Cr ferritic/martensitic steel according to an embodiment exhibits superior mechanical properties (see tables 2 and 3, FIGS. 1 and 2) compared to the general conventional high-Cr ferritic/martensitic steels, the high-Cr ferritic/martensitic steel according to an embodiment can be effectively used as a material for devices nuclear fuel cladding tube, duct or wire wrap in a sodium-cooled fast reactor, or a Generation IV reactor.

The present inventive technical concept will be explained in greater detail below based on the several examples which are not to be construed as limiting the present inventive concept.

Example 1 Preparation of High-Cr Ferritic/Martensitic Steels

As for experimental materials, 0.07 weigh % of carbon, 0.04 weight % of silicon, 0.428 weight % of manganese, 0.460 weight % of nickel, 9.10 weight % of chromium, 0.52 weight % of molybdenum, 0.20 weight % of vanadium, 0.05 weight % of tantalum, 0.20 weight % of niobium, 2.00 weight % of tungsten, 0.083 weight % of nitrogen, 0.0056 weight % of boron, and iron balance were processed in a Vacuum Induction Melting Furnace into a 30 kg of ingot. The ingot was retained at 1150° C. for 2 hours, and achieved hot rolling to a final thickness of 15 mm.

Heat treatment was then performed as follows.

To be specific, the alloy was normalized at 1050° C. for 1 hour, and was air-cooled.

After that, the normalized alloy was tempered at 750° C. for 2 hours and was air-cooled to form a high-Cr ferritic/martensitic steel.

The high-Cr ferritic/martensitic steel underwent additional heat treatment and cool working which were repeated successively for 2 to 4 times, and then underwent final heat treatment at 600˜800° C. for 1˜3 hours to form a final product of high-Cr ferritic/martensitic steel.

Example 2

A high-Cr ferritic/martensitic steel was produced with a method similar to that of Example 1, except that 0.069 weight % of carbon, 0.045 weight % of silicon, 0.445 weight % of manganese, 0.450 weight % of nickel, 8.97 weight % of chromium, 0.49 weight % of molybdenum, 0.203 weight % of vanadium, 0.02 weight % of tantalum, 0.20 weight % of niobium, 2.07 weight % of tungsten, 0.084 weight % of nitrogen, 0.0063 weight % of boron and iron balance were used as experimental materials.

Example 3

Except that 0.069 weight % of carbon, 0.049 weight % of silicon, 0.435 weight % of manganese, 0.447 weight % of nickel, 8.96 weight % of chromium, 0.49 weight % of molybdenum, 0.205 weight % of vanadium, 0.10 weight % of tantalum, 0.19 weight % of niobium, 2.01 weight % of tungsten, 0.086 weight % of nitrogen, 0.006 weight % of boron and iron balance were used as experimental materials, other processes are the same as example 1 to achieve high-Cr ferritic/martensitic steels.

Example 4

A high-Cr ferritic/martensitic steel was produced with a method similar to that of Example 1, except that 0.073 weight % of carbon, 0.054 weight % of silicon, 0.442 weight % of manganese, 0.451 weight % of nickel, 8.90 weight % of chromium, 0.50 weight % of molybdenum, 0.202 weight % of vanadium, 0.05 weight % of tantalum, 0.20 weight % of niobium, 3.00 weight % of tungsten, 0.084 weight % of nitrogen, 0.0067 weight % of boron and iron balance were used as experimental materials.

Example 5

A high-Cr ferritic/martensitic steel was produced with a method similar to that of Example 1, except that 0.068 weight % of carbon, 0.047 weight % of silicon, 0.44 weight % of manganese, 0.455 weight % of nickel, 9.03 weight % of chromium, 0.49 weight % of molybdenum, 0.2 weight % of vanadium, 0.05 weight % of tantalum, 0.20 weight % of niobium, 2.01 weight % of tungsten, 0.077 weight % of nitrogen, 0.0061 weight % of boron, 0.004 weight % of phosphorus and iron balance were used as experimental materials.

Example 6

A high-Cr ferritic/martensitic steel was produced with a method similar to that of Example 1, except that 0.07 weight % of carbon, 0.045 weight % of silicon, 0.427 weight % of manganese, 0.443 weight % of nickel, 8.83 weight % of chromium, 0.51 weight % of molybdenum, 0.194 weight % of vanadium, 0.05 weight % of tantalum, 0.20 weight % of niobium, 2.04 weight % of tungsten, 0.077 weight % of nitrogen, 0.006 weight % of boron, 0.009 weight % of phosphorus and iron balance were used as experimental materials.

Example 7

A high-Cr ferritic/martensitic steel was produced with a method similar to that of Example 1, except that 0.066 weight % of carbon, 0.046 weight % of silicon, 0.436 weight % of manganese, 0.450 weight % of nickel, 9.02 weight % of chromium, 0.49 weight % of molybdenum, 0.199 weight % of vanadium, 0.05 weight % of tantalum, 0.19 weight % of niobium, 2.04 weight % of tungsten, 0.076 weight % of nitrogen, 0.0059 weight % of boron, 0.016 weight % of zirconium and iron balance were used as experimental materials.

Comparative Example 1

Conventionally available ASTM Gr. 92 alloy was used.

(Compositions: 0.096 weight % of carbon, 0.060 weight % of silicon, 0.44 weight % of manganese, 0.19 weight % of nickel, 8.95 weight % of chromium, 0.48 weight % of molybdenum, 0.204 weight % of vanadium, 0.055 weight % of niobium, 1.9 weight % of tungsten, 0.045 weight % of nitrogen and iron balance)

Comparative example 2

Conventionally available HT9 alloy was used.

(Compositions: 0.192 weight % of carbon, 0.14 weight % of silicon, 0.490 weight % of manganese, 0.484 weight % of nickel, 12.05 weight % of chromium, 1.00 weight % of molybdenum, 0.304 weight % of vanadium, 0.022 weight % of niobium, 0.496 weight % of tungsten, 0.011 weight % of nitrogen and iron balance)

The compositions of the high-Cr ferritic/martensitic steels prepared by the Examples 1 to 7 and Comparative examples 1 and 2 are listed in Table 1 below.

TABLE 1 Compositions (weight %) C Si Mn Ni Cr Mo V Ta Nb W N B Example 1 0.07 0.04 0.428 0.460 9.10 0.52 0.20 0.05 0.20 2.00 0.083 0.0056 Example 2 0.069 0.045 0.445 0.450 8.97 0.49 0.203 0.02 0.20 2.07 0.084 0.0063 Example 3 0.069 0.049 0.435 0.447 8.96 0.49 0.205 0.10 0.19 2.01 0.086 0.006 Example 4 0.073 0.054 0.442 0.451 8.90 0.50 0.202 0.05 0.20 3.00 0.084 0.0067 Example 5* 0.068 0.047 0.44 0.455 9.03 0.49 0.20 0.05 0.20 2.01 0.077 0.0061 Example 6** 0.07 0.045 0.427 0.443 8.83 0.51 0.194 0.05 0.20 2.04 0.077 0.006 Example 7*** 0.066 0.046 0.436 0.450 9.02 0.49 0.199 0.05 0.19 2.04 0.076 0.059 Comparative 0.096 0.060 0.44 0.19 8.95 0.48 0.204 0.055 1.9 0.045 example 1 Comparative 0.192 0.14 0.490 0.484 12.05 1.00 0.304 0.022 0.496 0.011 example 2 *add 0.004 of P **add 0.009 of P **add 0.016 of Zr

<Experiment> Property Measurement of High-Cr Ferritic/Martensitic Steels (1) Measurement of Yield Strength and Tensile Strength

To measure the properties of high-Cr ferritic/martensitic steels prepared by Examples 1 to 7 and Comparative examples 1 and 2 at a high temperature, tensile test (ASTM E 8M-08) was conducted to measure yield strength and tensile strength, and the results are listed in Table 2 and FIG. 1.

TABLE 2 Yield strength (MPa) Tensile strength (MPa) Example 1 348 362 Example 2 331 341 Example 3 316 330 Example 4 350 372 Example 5 325 342 Example 6 319 342 Example 7 333 346 Comparative 272 292 example 1 Comparative 323 355 example 2

As illustrated in Table 2 and FIG. 1, the high-Cr ferritic/martensitic steels have 316˜350 MPa of yield strength and 330˜372 MPa of tensile strength. Compared to the general conventional high-Cr ferritic/martensitic steels (Gr. 92 alloy; Comparative example 1272 MPa of yield strength and 292 MPa of tensile strength), the high-Cr ferritic/martensitic steels according to an embodiment have greater than equal yield strength and tensile strength.

Therefore, the high-Cr ferritic/martensitic steels according to an embodiment have high yield strength and high tensile strength at a high temperature of 650° C., and can be used as nuclear fuel material for a Generation IV SFR which is used under severe conditions of high temperature and high amount of neutrons.

(2) Measurement of Creep Resistance

To measure the creep resistance of high-Cr ferritic/martensitic steels prepared according to Examples 1 to 7 and Comparative examples 1 to 2, rupture time was measured with 140 MPa stress intensity at a temperature of 650° C., and the result is listed in Table 3 and FIG. 2.

TABLE 3 Creep resistance (Time) Example 1 2321 Example 2 2178 Example 3 2104 Example 4 3233 Example 5 1833 Example 6 1839 Example 7 2440 Comparative 814 example 1 Comparative 148 example 2

As shown in Table 3 and FIG. 2, the high-Cr ferritic/martensitic steels according to an embodiment show 1833˜3233 rupture time under 140 MPa of stress intensity at 600° C. Compared to the general conventional high-Cr ferritic/martensitic steels (Gr. 92 alloy, Comparative example 1, rupture time: 814 hours; and HT9 alloy, Comparative example 2, rupture time: 148 hours), the high-Cr ferritic/martensitic steels according to an embodiment have superior creep resistance.

Therefore, the high-Cr ferritic/martensitic steels according to an embodiment have improved creep resistance and can be used as nuclear fuel materials for generation IV SFR which is used under sever conditions of high temperature and high amount of neutrons.

The foregoing exemplary embodiments and advantages are merely exemplary and are not to be construed as limiting the present inventive concept. The present teaching can be readily applied to other types of apparatuses. Also, the description of the exemplary embodiments of the present invention is intended to be illustrative, and not to limit the scope of the claims, and many alternatives, modifications, and variations will be apparent to those skilled in the art. 

1. A high-Cr ferritic/martensitic steel having an improved tensile strength and creep resistance which includes 0.04˜0.13 weight % of carbon, 0.03˜0.07 weight % of silicon, 0.40˜0.50 weight % of manganese, 0.40˜0.50 weight % of nickel, 8.5˜9.5 weight % of chromium, 0.45˜0.55 weight % of molybdenum, 0.10˜0.25 weight % of vanadium, 0.02˜0.10 weight % of tantalum, 0.15˜0.25 weight % of niobium, 1.5˜3.0 weight % of tungsten, 0.05˜0.12 weight % of nitrogen, 0.004˜0.008 weight % of boron and iron balance.
 2. The high-Cr ferritic/martensitic steel having an improved tensile strength and creep resistance which includes 0.04˜0.13 weight % of carbon, 0.03˜0.07 weight % of silicon, 0.40˜0.50 weight % of manganese, 0.40˜0.50 weight % of nickel, 8.5˜9.5 weight % of chromium, 0.45˜0.55 weight % of molybdenum, 0.10˜0.25 weight % of vanadium, 0.02˜0.10 weight % of tantalum, 0.15˜0.25 weight % of niobium, 1.5˜3.0 weight % of tungsten, 0.05˜0.12 weight % of nitrogen, 0.004˜0.008 weight % of boron, 0.002˜0.010 weight % of phosphorus and iron balance.
 3. The high-Cr ferritic/martensitic steel having an improved tensile strength and creep resistance which includes 0.04˜0.13 weight % of carbon, 0.03˜0.07 weight % of silicon, 0.40˜0.50 weight % of manganese, 0.40˜0.50 weight % of nickel, 8.5˜9.5 weight % of chromium, 0.45˜0.55 weight % of molybdenum, 0.10˜0.25 weight % of vanadium, 0.02˜0.10 weight % of tantalum, 0.15˜0.25 weight % of niobium, 1.5˜3.0 weight % of tungsten, 0.05˜0.12 weight % of nitrogen, 0.004˜0.008 weight % of boron, 0.01˜0.08 weight % of zirconium and iron balance.
 4. A preparation method of the high-Cr ferritic/martensitic steel of one of claims 1 to 3, the method comprising steps of: mixing and dissolving alloying elements to form an ingot (step 1); hot working such as hot forging, hot extrusion and hot rolling the ingot prepared at step 1(step 2); normalizing the hot worked ingot of step 2 and air cooling the ingot (step 3); and tempering the normalized alloy of step 3 and air cooling the alloy to prepare high-Cr ferrite/martensitic steels (step 4).
 5. The preparation method of claim 4, wherein the ingot at step 1 is prepared by vacuum inducing melting (VIM) method.
 6. The preparation method of claim 4, wherein the hot working at step 2 is performed after a heat treatment at 1100˜1200° C. for 0.5˜2 hours.
 7. The preparation method of claim 4, wherein the normalizing at step 3 is performed at 1000˜1100° C. for 0.5˜2 hours.
 8. The preparation method of claim 4, wherein the tempering at step 4 is performed at 600˜800° C. for 1˜3 hours.
 9. The preparation method of claim 4, after step 4, further comprising additional intermediate heat treatment at 600˜800° C. for 1˜3 hours, 2˜4 times of consecutive cold working, and final heat treatment at 600˜800° C. for 1˜3 hours.
 10. A core device of a Generation IV reactor using the high-Cr ferritic/martensitic steel according to one of claims 1 to
 3. 11. The core device according to claim 10, wherein the Generation IV reactor is a sodium-cooled fast reactor (SFR).
 12. The core device according to claim 10, wherein the core device is one selected from a group consisting of a nuclear fuel cladding tube, a duct and a wire wrap. 